1. Field of the Invention
The invention relates to a cooling device used for preventing temperature from rising in electric parts such as small-sized computers and the like, which generate heat. In particular, the invention relates to a cooling device provided with a heat pipe.
2. Description of the Related Art
It is known to cool semiconductor devices and the like, mounted on various kinds of electrical equipment such as personal computers and other equipment, by air in a casing of the equipment, and by attaching a cooling body to a semiconductor device.
In the case of cooling the semiconductor device with a cooling body attached to the semiconductor device, there is often employed a manner of dispersing heat to a heat transferring body without directly attaching a radiating fin to the semiconductor device and radiating the heat through the fin directly attached to the heat transferring body because of the semiconductor device is generally small. The heat generated by the semiconductor device to be cooled is generally transmitted to the heat transferring body and radiated through the fins. The heat transferring body is frequently made of the material which has superior heat conductivity such as aluminum material and copper material.
The heat transferring body attached to the semiconductor device may be referred to as the heat spreader in view of a heat spreading. There is shown in FIG. 29 an example of the heat spreader using metal as a conventional heat transferring body. A semiconductor device 102 to be cooled is packaged on a printed circuit board 105. On an upper surface of the semiconductor 102 is disposed a heat spreader 100 which is a heat transferring body made of metal. The heat transmitted to the heat spreader 100 is radiated from fins 40.
In order to connect a body to be cooled, such as the semiconductor device 102, to the heat spreader in a heat transferring manner, both are in direct contact with each other, or connected through a heat transferring medium 110. For example, the heat transferring medium 110 is made of a heat transferring grease, thereby lowering the heat transferring resistance.
Moreover, the fins 40 may be attached to the heat spreader 100 in order to spread the heat more effectively. It is desired that the fins 40 be disposed in the vicinity of the outside of a casing of the equipment mounting the semiconductor device 102. Accordingly, the heat spreader 100 and the fins 40 may be away from each other depending on the location of the semiconductor device 102. In this case, the heat spreader 100 and the fins 40 can be connected through a heat pipe. The heat pipe transmits the heat as follows. The heat pipe receives, at an absorbing side thereof, the heat which is transmitted through the material of a container constituting the heat pipe, thereby causing the working fluid to be evaporated to allow the vapor to move to a heat radiation side of the heat pipe. The heat pipe cools the vapor of the working fluid at the heat radiation side thereof to return it to a liquid phase state again. Then the liquefied working fluid is moved to the heat absorbing side again. In this way, the circulation of the working fluid subjected to the phase change causes the heat to be transferred.
During the circulation of the working fluid, the vapor is moved due to gas diffusion, and then the liquid is moved due to gravity and the capillary phenomenon. Wicks may be employed in order to utilize the capillary phenomenon. In a case where the heat is transmitted through a heat transferring solid body such as metal, the transmission of the heat is caused due to the temperature difference. Accordingly, the larger the temperature difference, the larger the quantity of the transmitted heat, whereas in the case of the heat pipe, the larger the vaporization latent heat of the working liquid or the larger the circulation rate of the working fluid, the larger the quantity of transmitted heat.
As a result, the heat pipe is capable of transmitting plenty of heat even if the temperature difference is small, provided that the temperature of the heat absorbing side is higher than the vaporization temperature of the working fluid and that the temperature of the heat radiation side is lower than the vaporization temperature of the working fluid. Therefore, the heat pipe is effective in a case where the temperature of the body to be cooled is required to be lowered substantially to the room temperature. In many cases, heat pipes are employed as not only the connection between the conductive metal body and the fins but also as the heat spreader. And heat pipes are also employed with the heat transferring metal body.
There is shown in FIG. 30 a heat spreader in which a heat pipe is embedded in the heat transferring body. A heat pipe 109 is incorporated into a heat transferring metal body 108, thereby reducing the weight of the heat transferring metal body 108 by the amount that the heat transferring metal body 108 is replaced by the heat pipe 109. Although this heat pipe is thick and short in length, the construction thereof is the same as that of the usual heat pipe, so it may be called the plate-like flat heat pipe.
Recently, equipment including electronic products have been improved with respect to function and are made small-sized and light-weight by using electronic parts such as semiconductor devices, which require the temperature to be limited to room temperature. In such equipment, it is advantageous that instead of the heat transferring metal body, the heat pipe is employed as the heat spreader.
The heat pipe has a disadvantage in that the heat absorbing side of the heat pipe must be located below the heat radiation side because the liquefied working fluid flows downwardly. For the purpose of eliminating this disadvantage it is proposed that the liquefied working fluid is guided by means of wicks. The wick, which has a capillary function, moves liquid by surface tension. For example, Japanese Patent Provisional Publication (Kokai) No. 7-208884 proposed that a plate-like heat pipe having block-like wicks each formed by numerous capillaries tubes are arranged so as to contact with upper and lower surfaces of the heat pipe.
The above-mentioned proposal is shown in FIG. 31. FIG. 31(a) is a longitudinal sectional view of a container 200 containing working fluid, and FIG. 31(b) is a cross sectional view thereof. First wicks 201 are disposed between a heat radiating wall 202 and a heat absorbing wall 203, with a space 204 formed therearound, and then second wicks 205 are disposed along the heat radiation wall 202 and the heat absorbing wall 203. Then as shown in FIG. 31(b), there are a plurality of the first wicks 201, which are arranged radially.
The first wick 201, having a strong capillary force, is formed into a block, whereas the second wick 205, having a weak capillary force, comprises slightly rough metal wire mesh, each arranged horizontally, are stacked one over another. Reference numeral 210 designates an exothermic body. Even if the exothermic body 210 is located above the container 200, which is in the so-called top heat mode, the working fluid in the liquid phase condensed at the heat radiating wall 202 located below and is transferred to the heat absorbing wall 203 located above by means of the first wick 201. Then, the working fluid evaporates into a gas phase at the heat absorbing wall 203 and reaches the heat radiating wall 202 through the space 204 from the second wick 205 having a rough mesh.
However, in the field of the electronic products and the like, the bodies to be cooled are, in many cases, small in size and high in exothermic density like the semiconductor devices. In the above-mentioned heat pipe, the portion of the heat absorbing wall with which the exothermic body contacts is also contacted by working fluid in the liquid phase that is sealed in the mesh of the metal wire gauze, like the non-contacting portion.
The portion of the heat absorbing wall with which the exothermic body contacts has a higher temperature than the other positions. Accordingly, the temperature difference in the working fluid is large and hence the heat flow rate is high. As a result, the working fluid in the liquid phase cannot overtake the demand of the heat absorbing wall at the above contacting portion, which causes the evaporated working fluid to be excessively heated. Although the boiling limit means a state in which there is no working fluid in the liquid phase, when the temperature reaches the boiling limit, the heat of the heat absorbing surface can be transferred merely as sensible heat without being transferred as latent heat, which degrades the heat transferring efficiency.
According to the arrangement of the bodies to be cooled and the service condition of the electric equipment and the mounting of the bodies to be cooled, the bodies to be cooled are not necessarily arranged only on a horizontal surface. Accordingly, in the case of the portable equipments, they are arranged on an inclined surface. In such a state, the working fluid is prevented from being moved by gravity, which significantly lowers the heat transferring function of the heat pipe.
Moreover, as a result of reducing the size-of the equipment, a number of the bodies to be cooled are densely arranged in many cases requiring a number of cooling devices in narrow spaces in order to cool the equipment, which brings about the inconvenience of making the equipment further complicated. Under these circumstances it is desired that the cooling device be capable of dealing with semiconductor devices to be cooled having high exothermic density and capable of a variety of arrangements such as the inclined arrangement and the dense arrangement of the bodies to be cooled.
The invention has been made to solve the above problems, it is, therefore, an object of the invention to provide a cooling device which is capable of dealing effectively with a plurality of the bodies to be cooled densely arranged without significantly degrading the heat transferring efficiency even if the bodies to be cooled are high in exothermic density, and without significantly lowering the heat transferring function of the heat pipe even if the heat pipe is inclined.